The present invention relates to an image-reading apparatus, which read an image residing on a document by means of a photoelectronic converting element.
The image-reading apparatus has been employed as an image-inputting device for such an apparatus of an image scanner, a digital copier, a facsimile, etc. In order to achieve the image-reading operation, such the image-reading apparatus employs the photoelectronic converting element, such as the CCD (Charge Coupled Device), etc., for converting the image residing on the document to the electronic signals.
In the image-reading apparatus, which employs a one-dimensional image sensor, such as a one-dimensional CCD, etc., the image-reading operation in a main-scanning direction is performed by the electronic scanning action of the one dimensional image sensor, while the image-reading operation in a sub-scanning direction is performed by the mechanical scanning action for moving the one dimensional image sensor or the document in the sub-scanning direction. In such the configuration, by changing the sub-scanning velocity, it is possible to change the magnification factor in the sub-scanning direction. For instance, the reduction of the image size in the sub-scanning direction can be achieved by reading the original image with an increased sub-scanning velocity.
To increase the scanning velocity in the sub-scanning direction, however, the driving motor, having a capability of moving the document or the one-dimensional image sensor at a sufficiently high velocity, should be employed. Accordingly, it has been a problem that such the high-performance driving motor has been very expensive, resulting in an increase of the manufacturing cost of the image-reading apparatus. In addition, it has been also a problem that the high-speed operation of the driving motor causes the raise of malfunction rate and the generation of the motor noise.
To overcome the abovementioned problems, there has been well known, for instance, a method of making the sub-scanning velocity constant irrespective of the magnification factor for reading the original image by increasing the index period, as set forth in Tokkai 2000-332959. Incidentally, the term of the “index period” means a time interval required for reading one line of the document in the main-scanning direction. Further, as set forth in Tokkaihei 6-38028, Tokkaihei 8-152678 and Tokkaihei 10-23224, there has been well known a method of reducing the image size of the document in the sub-scanning direction by changing the line-thinning rate in accordance with the reading magnification factor for the document, without employing the driving motor having a high-speed rotating capability. Incidentally, the term of the “line-thinning” is to thin out a number of lines to be read in the sub-scanning direction so as to reduce the image size of the document. Further, hereinafter, an operation for thinning out one line from every “n” lines (“n” is a positive integer being equal to or larger than 2) is defined as a “1/n line-thinning operation”.
When the index period is set at a longer time, however, it has bee necessary to increase the storing capacity of the storage in which the image-signals concerned are stored. Further, since the time necessary for accumulating the image-signal charges in the CCD becomes large, it has been necessary to take such a countermeasure of considerably lowering the gain of the pre-amplifier, or reducing the brightness of the illumination lamp for irradiating the document. On the other hand, it has been another problem that the image quality is considerably deteriorated, when the line-thinning operation is conducted in accordance with the reading magnification factor for the document.